Conventionally, a fixing device for use in an electrophotographic apparatus utilizing an electrophotographic process, such as a copying machine, a facsimile machine, or a printer, has been equipped with, for example, a fixing roller 101 and a pressure roller 102 which is pressed against the fixing roller 101, as illustrated in FIG. 9. Inside the fixing roller 101, a heater lump 103 as a heat source is provided, so that a surface of the fixing roller 101 is heated from inside.
The fixing roller 101 is composed of a fixing roller core 101a and a rubber layer 101b which covers the fixing roller core 101a.
The pressure roller 102 is composed of a pressure roller core 102a and a rubber layer 102b which covers the pressure roller core 102a.
The fixing device with these components is arranged so that a sheet of paper P (recording material) carrying thereon an image formed with pre-fixed toner T is transported to a region (fixing region) 108 where the fixing roller 101 heated to a desired set temperature and the pressure roller 102 come into contact, and the toner T is fused with heat and fixed on the sheet of paper P. Such a fixing device in which the heat source such as the heater lump 103 or the like is provided in the fixing roller 101 to heat the surface of the fixing roller 101 is referred to as a fixing device of inside heating type.
In the case where the toner carried on the sheet of paper P does not have a good releasing property, that is, for example, in the case of color toner, adhesion of the toner to the surface of the fixing roller 101, namely, an offset phenomenon, occurs on the fixing of the toner. Therefore, the fixing device is equipped with an oil applying unit 105 for applying oil on the surface of the fixing roller 101. The oil applying unit 105 is arranged so that oil 104 for preventing such an offset phenomenon is applied to the surface of the fixing roller 101 through a pair of oil applying rollers 107.
Incidentally, when the fixing device is of a type with an internal heating unit, it takes a long time to heat the surface of the fixing roller 101 to a set temperature (the toner fixation temperature) since the fixing roller 101, which has a great thermal capacity, is heated from the inside by the heater lump 101. (The period of time required to heat the fixing roller 101 to a set temperature is hereinafter referred to as the warm-up period.
As a fixing device which requires a shorter warm-up period, the Japanese Publications for Laid-Open Patent Application No. 50-62448/1975 (Tokukaisho 50-62448), No. 51-70648/1976 (Tokukaisho 51-70648), and No. 52-131731/1977 (Tokukaisho 52-131731) disclose a fixing device in which a surface of a fixing roller is heated by a heat roller (heat source) which is provided in contact with the surface of the fixing roller, that is, a fixing device of outside heating type. This fixing device of outside heating type has an excellent thermal efficiency since the heat source is in contact with the surface of the fixing roller, and hence the warm-up period required to heat the fixing roller to a set temperature necessary for fixation is considerably reduced.
Recently, the Japanese Publications for Laid-Open Patent Applications No. 63-313182/1988 (Tokukaisho 63-313182), No. 4-358186/1992 (Tokukaihei 4-358186), and No. 5-2349/1993 (Tokukaihei 5-2349) have disclosed a fixing device arranged so that a sheet of paper carrying a pre-fixed toner image is heated through a thin endless film (endless belt) which moves in synchronization with the sheet of paper, so that the toner is fixed. (Such a fixing device is hereinafter referred to as a fixing device of film heating type.)
The fixing device of film heating type is arranged, for example, as follows: as illustrated in FIG. 10, a sheet of paper P carrying an image formed with pre-fixed toner T is transported to a region where (1) a fixing belt 201 which is formed in an endless belt form and is stretched in a recording material transporting direction and (2) a pressure roller 203 come into contact, so that the image of the pre-fixed toner T is fixed on the sheet of paper P.
A heating body 202 is provided vis-a-vis the pressure roller 203 with the fixing belt 201 therebetween so that a fixation region 204 formed between the fixing belt 201 and the pressure roller 203 is heated. Therefore, the paper P transported through the fixation region 204 is heated with thermal energy from the heating body 202, and the toner carried on the paper P is fused and fixed thereon.
The fixing device as described above is arranged so that the sheet of paper P on which the toner has been fixed is naturally cooled down when it is transported to a downstream side of the fixing belt 201 (this cooling phenomenon is hereinafter referred to as self-cooling effect), so that the paper P separates from the fixing belt 201. Therefore, in the fixing device of film heating type, the toner-fixed paper P is cooled down when it comes to the downstream part of the fixing belt 201, and hence no adhesion of the toner to the fixing belt 201 occurs. As a result, application of oil for preventing an offset phenomenon is unnecessary. As the fixing belt 201, a belt which is made of a heat-resistant resin such as polyimide and is covered with a material having a good releasing property, such as a fluorocarbon resin, is usually used.
In the fixing device of film heating type, the fixing belt 201 may have a small thermal capacity, thereby causing the sheet of paper with the toner T to be cooled by radiation of heat immediately after passing through the fixation region 204 as a heated region. This causes the cohesive force of the toner T to increase, thereby causing the adhesive force to the fixing belt 201 to relatively decrease as the toner T is cooled down. Therefore, the offset can be prevented in principle.
The Japanese Publication for Laid-Open Patent Application No.6-318001/1994 (Tokukaihei 6-318001) discloses a fixing device of film heating type in which heating of a film (belt) with a small thermal capacitance is carried out on an upstream side to a fixing nip region. Therefore, the film is naturally cooled down in the fixing nip region (self-cooling effect) since heat of the film is taken by toner and paper. As a result, a temperature of the fused toner does not become extraordinarily high, and offset is prevented.
However, the aforementioned conventional fixing devices of various types have the following problems.
(I) Problems of the Fixing Device of Inside Heating Type
In order to achieve sufficient fixation, it is necessary to increase a heat quantity for fixation. In the color printing, in particular, in which multi-layer toner fixation is conducted, a great amount of thermal energy is required. As a method for increasing the heat quantity for fixation, there are two alternatives: raising the temperature of the fixing roller; and widening the fixing nip region which is a region where the fixing roller and the pressure roller come into contact.
However, in the case where the temperature of the fixing roller is raised, the offset phenomenon more likely occurs, while a service life of the fixing roller is shortened.
On the other hand, as a method for widening the nip width of the fixing nip region, there are following alternatives: (a) thickening the rubber layer 101b covering the fixing roller 101; (b) increasing the pressing force of the pressure roller 102; and (c) thickening the rubber layer 102b covering the pressure roller core 102a of the pressure roller 102.
In the case of the method (a), the rubber layer 101b becomes thicker, and this causes thermal conductivity to deteriorate. This further causes a drawback in that the period of time required to achieve the fixation temperature of the fixing device, that is, the warm-up period, becomes longer. Besides, there occurs another drawback in that a temperature of an interface between the fixing roller core 101a and the rubber layer 101b of the fixing roller 101 rises, thereby as a result causing deterioration of the rubber layer 101b and separation of the same from the fixing roller core 101a.
In the case of the method (b), it is necessary to thicken the fixing roller core 101a of the fixing roller 101 so as to prevent deformation of the fixing roller 101 due to the pressure of the pressure roller 102. In this case as well, there occurs a drawback in that the thermal conductivity deteriorates, thereby causing the warm-up period to become longer.
In the case of the method (c), the sheet of paper P is pressed against the fixing roller 101 by the pressure roller 102 so as to conform with the shape of the fixing roller 101. As a result, the paper P is caught by the fixing roller 101, and it becomes difficult to separate the paper P from the fixing roller 101.
In the case of the method (c), to prevent the paper P from being caught by the fixing roller 101 and to separate the paper P from the fixing roller 101 without failure, it is necessary to provide a separating claw 106 in contact with the fixing roller 101, as illustrated in FIG. 9. However, in this case, the following problems occur:
(i) the device has a complicated arrangement, and a manufacturing cost of the device rises; PA1 (ii) the surface of the fixing roller 101 is scarred by the separating claw 106; PA1 (iii) an image is distorted due to the contact of the separating claw 106 to the surface of the paper on which the image is held. In the case of color printing, in particular, the distortion of the image is conspicuous; and PA1 (iv) the sheet of paper P curls due to the separating claw 106. PA1 (i) a complicated system is needed to apply the oil 104 evenly over the fixing roller 101, causing the cost of the device to rise; PA1 (ii) due to the oil 104, the rubber layer 101b of the fixing roller 101 deteriorates and swells, causing the service life of the fixing roller 101 to become shorter; PA1 (iii) spill of the oil 104 soils the device and reversely affect other parts of the apparatus; PA1 (iv) the oil 104 soils the sheet of paper P, thereby soiling hands of the user and making him/her feel unpleasant; PA1 (v) in the case where OHP is used as the sheet of paper P, the oil 104 soils the surface of the OHP, thereby lowering the transparency of the OHP; and PA1 (vi) regular maintenance such as supply of the oil 104 is required, and this makes the device not user-friendly.
In addition, in the fixing device, temperature detecting means such as a thermostat or a temperature fuse is provided near the fixing roller, so that it serves as a security device in the case where a trouble occurs to temperature controlling means or the like, causing a temperature of the fixing roller to extraordinarily rise. However, in the case where the temperature detecting means is provided in contact with the fixing roller so as to detect troubles in an early stage, there occurs a drawback in that the surface of the fixing roller is scarred. Besides, in the case where the temperature detecting means is provided not in contact with the fixing roller so that the fixing roller should not be scarred, the response of the temperature detecting means to an extraordinary state is delayed, thereby causing a delay in preventing the temperature of the fixing roller from rising. This results in that damage to the fixing device and the apparatus incorporating the fixing device is aggravated.
Furthermore, in the case where the oil 104 for preventing offset is applied to the surface of the fixing roller 101, the following drawbacks occur:
(II) Problems of the Fixing Device of Outside Heating Type
Since no heat is supplied in the fixing nip region, heat of the surface of the fixing roller is taken by the paper and the toner and the temperature of the surface of the fixing roller drops in the fixing nip region. As a result, a temperature gradient is caused between the upstream end (entrance) and the downstream end (exit) of the fixing nip region. This phenomenon is hereinafter referred to as self-cooling effect.
The temperature gradient due to the self-cooling effect considerably varies with the types of paper, an ambient temperature, and the number of sheets of paper which have been transported therethrough. For example, the temperature gradient is sharper as the paper is thicker, while the temperature gradient is sharper as the ambient temperature is lower.
Therefore, the conventional fixing device of outside heating type is necessarily arranged as follows: a temperature of the fixing roller just after being heated by the outside heating roller, that is, a temperature of a surface portion of the fixing roller on the upstream side to the fixing nip region with respect to the rotation direction of the fixing roller, is detected by the temperature detecting means, and the outside heating roller is controlled so that the temperature thus detected is converged to the set temperature. However, in the fixing device of outside heating type, as described before, due to the self-cooling effect, the temperature gradient in the fixing nip region varies with fixing conditions. Therefore, there occurs a drawback in that it is difficult to achieve a stable fixing property at all times.
Furthermore, in the case where sheets of paper with a narrower width than the width of the fixing roller, such as postcards or envelopes, are continuously transported through the fixing nip region, heat is not taken from portions of the fixing roller which do not touch the sheets of paper, and the surface temperature of the portions extraordinarily rises. Therefore, there occur problems of rise of a temperature inside an apparatus incorporating the fixing device, and adhesion of toner to the fixing roller, that is, an offset phenomenon (so-called high-temperature offset phenomenon).
In the case where toner with an inferior releasing property is used in the fixing device of outside heating type, offset, that is, adhesion of toner over the fixing roller surface, occurs as in the case of the fixing device of inside heating type. Therefore, the oil applying system is required so as to apply oil to the surface of the fixing roller. As a result, as in the case of the fixing device of inside heating type, various drawbacks occur due to the application of the offset-prevention oil to the fixing roller.
(III) Problems of the Fixing Device of Film Heating Type
The fixing device of film heating type has an advantage in that application of oil for preventing offset is not required. Besides, since a film which is used as fixing means of the fixing device has a small thermal capacity as compared with a fixing roller, the warm-up period can be shortened.
However, as illustrated in FIG. 10, since the fixing belt 201 is shaped in an endless belt form, systems for controlling tension applied to the fixing belt 201, preventing the fixing belt 201 from meandering, and preventing wrinkles from occurring to the fixing belt 201 due to thermal expansion, are required. Therefore, a driving system for the fixing belt 201 becomes very complicated, thereby raising the cost of the device.
Besides, since the fixing belt 201 is formed thin so as to have a small thermal capacitance, its service life is shorter than that of the fixing roller in a roll shape. Particularly, since the fixing belt 201 rubs against the heating body 202, the fixing belt 201 is worn away, and a service life of the fixing belt 201 becomes shorter. Besides, in the case where the rotation of the fixing belt is speeded up so as to speed up the printing, the service life of the fixing belt becomes further shorter. Therefore, it cannot be applied to an electrophotographic apparatus whose printing speed is high.
By thickening the fixing belt 201, the aforementioned problems can be solved to some extent. However, the thermal capacity of the fixing belt 201 increases also by thickening the fixing belt 201. This causes the warm-up period to become longer, and hence the advantage of the fixing device of film heating type, in that a shorter warm-up period is required, is lost.
Furthermore, in the case where the fixing belt 201 has a greater thermal capacity, the toner on the sheet of paper cannot be sufficiently cooled after the fixation until it becomes solid, and as a result offset likely occurs. Therefore, to prevent the offset phenomenon, cooling means such as ventilation means is required on the downstream side in the transport direction of the fixing belt 201 so as to cool the toner on the paper after the fixation until it becomes solid. In this case, the thermal efficiency deteriorates, and the device becomes complicated and bulky.